Microneedles are small needles, typically in the range of from 1 μm (micron) to 3 mm long and from 10 nm to 1 mm in diameter at their bases, although the ranges can be wider, for instance up to 10 mm long and 2 mm at their bases. Microneedles typically have applications in biomedical devices, for instance for transdermal drug delivery. Existing microneedle fabrication techniques tend to produce microneedles that are too soft (made of polymeric materials), too brittle (made of silicon) or too costly, or tend to be too unreliable. For transdermal drug delivery applications, where penetration of the outer skin (stratum corneum) is necessary, there are minimum requirements for the strength and ductility of a microneedle. Prices should be low, as microneedles are usually single-use products.
European Patent Application Publication No. EP-A1-1,088,642, published on 4 Apr. 2001 in the name of Becton Dickinson & Co. describes a method of fabricating an array of solid microneedles by moulding. A silicon master mould member with a recessed surface is placed into a mould cavity. A plastic material is pumped into the mould cavity. Microneedles are formed in the recesses in the master mould member.
European Patent Application Publication No. EP-A1-1,287,847, published on 5 Mar. 2003 in the name of Lifescan, Inc. describes a method of fabricating hollow microneedles by plastic injection moulding. The mould is made of two parts. The top part has a conical recess within its moulding surface. One of the top and bottom parts has a protrusion extending to the moulding surface of the other part for forming the needle lumen.
U.S. Pat. No. B1-6,334,856, issued on 1 Jan. 2002 to Allen et al. describes various ways of making arrays of hollow microneedles. In one example masks are formed on the tips of solid microneedles of a silicon microneedle array, a layer of silicon dioxide or metal is coated onto the microneedle array, and the silicon is etched away to leave a hollow microneedle array of metal or silicon dioxide. In another example a layer of epoxy is cast onto an array of solid silicon microneedles. The level of the epoxy is reduced to below the tips of the microneedles. The silicon array is removed, leaving an epoxy secondary mould. A Ti—Cu—Ti seed layer is splutter-deposited onto the epoxy secondary mould and Ni—Fe electroplated onto the seed layer. The epoxy layer is then removed, leaving an array of hollow metal microneedles.
U.S. Pat. No. B1-6,379,324, issued on 30 Apr. 2002 to Gartstein et al. describes various ways of making arrays of hollow microneedles. One way involves self-moulding a polymer film over micro-pillars through heating. A second approach is to place a polymer film over micro-pillars, heat the film and press it down over the micro-pillars using a recessed plate. A third way is to heat a plastic film in the lower part of a mould and to bring the upper part of the mould down onto the lower part. The upper part of the mould has micro-recesses, with micro-pillars protruding from their centres. As the upper part of the mould comes down, the lower parts of the micro-pillars displace the plastic of the plastic film up into the micro-recesses.